1. Field of the Invention
The present invention relates to numerical control device for controlling the movement of a movable member in accordance with numerical control data.
2. Description of the Prior Art
A numerical control device can control movement of a movable member with high accuracy. However, if a pulse distributing circuit or a servomotor drive circuit in the numerical control device fails to properly operate, the movable member may move at an overspeed. Accordingly, in such a case, it is not only impossible to properly position the movable member, but also a tool on the movable member could be brought into engagement with a workpiece at a rapid feed rate, resulting in breakage of the tool, fault of the workpiece and the like.
In order to avoid this, abnormality of a servo-mechanism has been conventionally detected to immediately stop movement of the movable member by checking whether a content of a differential counter exceeds a predetermined amount. This predetermined amount has been set to be relatively larger so as not to generate an abnormal signal at a normal rapid feed rate of the movable member. Accordingly, even if the movable member initiates to move at an overspeed during its feed movement, the abnormal signal cannot be immediately generated. It was therefore difficult to perfectly avoid engagement of the tool with the workpiece at the overspeed.